Sperm Competition and the Evolution of Animal Mating Systems

"Parker was invited to a US symposium organised by Bob Smith. Bob brought together everyone in the field and got them to write essays on key topics. This is a set of reviews on promiscuity and sperm competition across the animal kingdom. It was the first broad overview and recognition that sperm competition and promiscuity were ubiquitous. Smith’s own chapter on human sperm competition was exciting, stimulating and controversial. Until that point, we were rather reserved about these things. The implicit assumption was that female humans were monogamous, even though males were not. There’s a section on testes size in primates. It turns out that the relative size of testes in any animal group strongly correlates with the degree of promiscuity in that species. Therefore, gorillas have tiny testes for their body size, whereas chimpanzees have enormous ones. In gorilla societies, you have huge silverback males who can afford to produce relatively few sperm because they don’t need to compete. By contrast, male and female chimpanzees copulate with everyone in the troupe, every day. The males need huge quantities of sperm to do that. Exhausting, but exhilarating. What Bob Smith did was to fit humans into the data for primates. Humans are at the lower end of the scale. We’re not as exciting as chimpanzees but not as dull as gorillas."